A study on the mechanism of high-lift generation by an airfoil in unsteady motion at low reynolds number

The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re=100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the first translation, rotation and the second translation in the direction opposite to the first. The rotation and the second translation in this motion are expected to represent the rotation and translation of the wing-section of a hovering insect. The flow structure is used in combination with the theory of vorticity dynamics to explain the generation of unsteady aerodynamic force in the motion. During the rotation, due to the creation of strong vortices in short time, large aerodynamic force is produced and the force is almost normal to the airfoil chord. During the second translation, large lift coefficient can be maintained for certain time period and , the lift coefficient averaged over four chord lengths of travel, is larger than 2 (the corresponding steady-state lift coefficient is only 0.9). The large lift coefficient is due to two effects. The first is the delayed shedding of the stall vortex. The second is that the vortices created during the airfoil rotation and in the near wake left by previous translation form a short “vortex street” in front of the airfoil and the “vortex street” induces a “wind”; against this “wind” the airfoil translates, increasing its relative speed. The above results provide insights to the understanding of the mechanism of high-lift generation by a hovering insect. The project supported by the National Natural Science Foundation of China (19725210)     

受限分散体系中粒子扩散特性的研究

当微小粒子被限制在两个距离很近的平行平面之间时，其扩散将不同于它在无限空间中的自由扩散. 利用光镊和CCD图像相关处理系统，直接测量了直径1μm的聚苯乙烯小球在不同高度，一定时间间隔内，沿着平行玻片表面方向上布朗运动的距离，并由此得到了样品池上下两个平行表面对分散体系分散相小球扩散特性的影响. 还利用小球偏离物镜焦平面不同距离时的成像状况，用相关运算法确定了小球在纵向上的位置，实现了小球扩散特性与小球在样品池中的纵向位置的关系的研究.     

A new criterion for occurrence of strick-slip motion in drive mechanism

This paper, using Karnopp's model of friction force and phase plane method, studies the stick-slip motion of the flexble drive mechanism. It is explained that a sudden drop of friction force is the essential source of stick-slip motion when the sliding is impending. A new criterion for occurrence of stick-slip motion is established. The stick-slip region and the stable region in a parameter plane are separated by a critical parameter curve. Moreover, for the stick-slip motion of the flexible drive mechanism without viscous damping, a parameter expression is obtained. The results may be used in design of the flexible drive mechanism.     

Two-phase damping for internal flow: Physical mechanism and effect of excitation parameters

Two-phase flow induced-vibration is a major concern for the nuclear industry. This paper provides experimental data on two-phase damping that is crucial to predict vibration effects in steam generators. An original test section consisting of a tube subjected to internal two-phase flow was built. The tube is supported by linear bearings and compression springs allowing it to slide in the direction transverse to the flow. An excitation system provides external sinusoidal force. The frequency and magnitude of the force are controlled through extension springs. Damping is extracted from the frequency response function of the system. It is found that two-phase damping depends on flow pattern and is fairly proportional to volumetric fraction for bubbly flow. Measurements are completed by the processing of high-speed videos which allow to characterize the transverse relative motion of the gas phase with respect to the tube for bubbly flow. It is shown that the bubble drag forces play a significant role in the dissipation mechanism of two-phase damping.     

Motion generation of RCCC mechanism using numerical atlas

This article presents a synthesis method for the motion generation of RCCC mechanism using the numerical atlas. Based on the Fourier series theory, the geometrical model of the rigid body rotation angle of RCCC mechanism is established. Through harmonic analysis, the rotation angle output characteristics were observed to be dependent on the rigid body rotation angle operator. Subsequently, two numerical atlas databases are constructed. The theoretical formulas to calculate the real size and installing dimensions were proposed. On the basis of the theoretical formulas and two numerical atlas databases, the problem of rigid body guided synthesis of RCCC is solved.     

机抖激光捷联惯导系统静态圆锥运动形成机理

以三轴机械抖动激光陀螺捷联惯导系统为研究对象,利用数值仿真方法,详细讨论了静态环境条件下圆锥运动的形成机理以及机械抖动激光陀螺的惯性参数对惯导系统圆锥运动的影响规律。研究认为,在静态环境条件下,惯导系统的质量分布特性不是产生圆锥运动的原因;惯导系统在静态环境条件下出现的圆锥运动是由于机械抖动陀螺抖动机构的三类不平衡——静不平衡、动不平衡和配不平衡造成的。     

A numerical study of flow through wavy-walled channels

A numerical procedure is developed for the analysis of flow in a channel whose walls describe a travelling wave motion. Following a perturbation method, the primitive variables are expanded in a series with the wall amplitude as the perturbation parameter. The boundary conditions are applied at the mean surface of the channel and the first-order perturbation quantities are calculated using the pseudospectral collocation method. Although limited by the linear analysis, the present approach is not restricted by the Reynolds number of the flow and the wave number and frequency of the wavy-walled channel. Using the computed wall shear stresses, the positions of flow separation and reattachment are determined. The variations in velocity and pressure with frequency of excitation are also presented. © 1998 John Wiley & Sons, Ltd.     

Experimental study on dynamics of an oblique-impact vibrating system of two degrees of freedom

The paper presents a detailed experimental study of an oblique-impact vibration system of two degrees of freedom. The primary objective of the study is to verify the hypothesis of instantaneous impact in the oblique-impact process of two elastic bodies such that the incremental impulse method works for computing the nonlinear dynamics of the oblique-impact vibrating systems. The experimental setup designed for the objective consists of a harmonically excited oscillator and a pendulum, which obliquely impacts the oscillator. In the study, the dynamic equation of the experimental setup was established first, and then the system dynamics was numerically simulated by virtue of the incremental impulse method. Afterwards, rich dynamic phenomena, such as the periodic vibro-impacts, chaotic vibro-impacts and typical bifurcations, were observed in a series of experiments. The comparison between the experimental results and the numerical simulations indicates that the incremental impulse method is reasonable and successful to describe the dynamics during an oblique-impact process of two elastic bodies. The study also shows the limitation of the hypothesis of instantaneous impact in an oblique-impact process. That is, the hypothesis only holds true in the case when the impact angle is not too large and the relative approaching velocity in the normal direction is not too low. Furthermore, the paper gives the analysis of the tangential rigid-body slip on the contact surface in the case of a large impact angle, and explains why there exist some discrepancies between the numerical simulations and the experimental results.     

Numerical study of bubble and particle motion in a turbulent boundary layer using proper orthogonal decomposition

We have studied the motion of bubbles and particles in the near-wall region of a turbulent boundary layer, to investigate the influence of the unsteady turbulent structure. The velocity field was computed using Proper Orthogonal Decomposition (POD), and the trajectories of bubbles and particle have been computed by integrating their equation of motion. We have used this to investigate the roles, and the relative importance, of the different forces acting on bubbles and particles, We find that the unsteady turbulent structure plays an important role in the preferential accumulation of bubbles and particles. The accumulation of bubbles depends on a rather complicated interaction between the pressure gradient and the lift force; neither is sufficient, acting on its own, to explain the strong accumulation observed when they act together.     

球体入水空泡演变和运动特性影响试验研究

为了探究表面粗糙度对球体入水空泡演变及运动特性的影响,基于实验室开放水槽试验系统,选取了5种表面粗糙度的球体,使用高速摄像机记录入水过程,并得到了各个球体的入水空泡、喷溅的演变过程以及运动特性的变化。发现入水空泡和喷溅的闭合都会给球体一个负方向的加速度。通过对比不同表面粗糙度球体的位移、速度、加速度曲线,发现表面粗糙度最大的球体在砰击阶段结束后,其速度会明显小于其他球体,并且表面粗糙度对球体运动的影响主要体现在入水早期。分析了上述各球体的入水空泡闭合后,与自由面相连的空泡的收缩运动,发现其收缩速度和加速度曲线均会出现极大值点,呈现出球体表面粗糙度越大出现得越早的趋势。

     

The motion of a small bubble in waves

The motion of a single spherical small bubble due to buoyancy in the ideal fluid with waves is investigated theoretically and experimentally in this article. Assuming that the bubble has no effect on the wave field, equations of a bubble motion are obtained and solved. It is found that the nonlinear effect increases with the increase of the bubble radius and the rising time. The rising time and the motion orbit are given by calculations and experiments. When the radius of a bubble is smaller than 0.5mm and the distance from the free surface is greater than the wave height, the results of the present theory are in close agreement with measurements. The project supported by the National Natural Science Foundation of China     

The mechanics of short rod-like molecules in tension

The rapid development of single molecule experimental techniques in the last two decades has made it possible to accurately measure the force–extension response as well as the transverse fluctuations of individual rod-like macromolecules. This information is used in conjunction with a statistical mechanical model based on the treatment of the molecule as a fluctuating elastic rod to extract its bending and extension moduli. The models most commonly used to interpret the experimental data assume that the magnitude of the Brownian fluctuations are independent of the length of the macromolecule, an assumption that holds only in the asymptotic limit of infinitely long rods, and is violated in most experiments. As an alternative, we present a theoretical treatment of a finite length, fluctuating rod and determine its mechanical behavior by measuring the transverse Brownian fluctuations under the action of large stretching forces. To validate our theory, we have applied our methods to an experiment on short actin filaments whose force–extension relation is difficult to measure, but whose transverse deflections can be captured by current microscopy techniques. An important consequence of the short contour lengths is that the boundary conditions applied in the experiment affect the fluctuations and can no longer be neglected as is commonly done when interpreting data from force–extension measurements. Our theoretical methods account for boundary conditons and can therefore be deployed in conjunction with force–extension measurements to obtain detailed information about the mechanical response of rod-like macromolecules.     

The control of a permanent rotational motion and equilibrium position of a spacecraft

This article has adopted an analytical method to obtain a non-linear control law to reach the exponential asymptotic stablity of the permanent rotational motion of a spacecraft. The control moments achieving this rotational motion are obtained. The control moments to establish exponential asymptotic stablity of the mentioned motion are obtained as non-linear functions of the phase coordinates of the spacecraft. The general solution of the equations of perturbed motion is derived. Furthermore, analysis and numerical simulation study of this solution are presented. For numerical examples the time needed for control is calculated. An equilibrium position of the spacecraft is proved to be exponentially asymptotically stable as a special case of the above-studied problem.     

A cellular scale numerical study of the effect of mechanical properties of erythrocytes on the near-wall motion of platelets

The effect of mechanical properties of erythrocytes on the near-wall motion of platelets was numerically studied with the immersed boundary method. Cells were modeled as viscous-fluid-filled capsules surrounded by hyper-elastic membranes with negligible thickness. The numerical results show that with the increase of hematocrit, the near-wall approaching of platelets is enhanced, with which platelets exhibit larger deformation and orientation angle of its near-wall tank-treading motion, and the lateral force pushing platelets to the wall is increased with larger fluctuation amplitude. Meanwhile the near-wall approaching is reduced by increasing the stiffness of erythrocytes.     

The mechanism of violent condensation shocks

In direct-contact steam condensers, violent condensation shocks (VCSs) occur at low steam flow rates. This phenomenon was also observed in the pressure-suppression system of nuclear boiling water reactors. Thus, the phenomenology of condensation in this type of condenser has been investigated. The main design feature of the Plexiglas test apparatus used here is that the events of rapid steam-water condensation are observed in a sectional view instead of an external view. This allows the phenomena at the phase interfaces to be observed in detail. In our experiments we found that a characteristic feature of VCSs is the appearance of considerable entrainment inside so-called steam pockets, which is characterized by atomization and is correlated to the extremely high rate of condensation. This avalanche-like increase in water atomization is induced by the Kelvin—Helmholtz instability and occurs because of the increasing steam flow velocity along the freely movable surface of water, primarily in the bottle-neck of the steam pocket. Detailed examination of the experimental data shows that the entrainment causes the temporary high condensation rates, which were previously observed but not understood. This is in agreement with the conditions of a sonic steam jet blowing into a subcooled pool of water where entrainment stimulates the condensation process in a similar way. The extraordinarily high condensation rates in the steam pocket induce very high steam velocities in the vent pipe, so that the entrainment often propagates outside the steam pocket in the vent pipe. We conclude that the initiating mechanism of VCSs is this self-amplifying feedback process which lasts until the initiating steam pocket has disappeared. The induced state of rapid condensation outside the steam pocket decays with a time constant in the range of 0.1 s.     

爆炸烟云中粒子运动的数值模拟

以10 kg TNT爆炸装置在开阔无风地带爆炸为背景,使用Autodyn计算得到爆炸后1 ms时的爆轰产物状态,为爆炸烟云中粒子运动的数值模拟研究提供了可靠的源项几何模型和物理参数;而后使用GAMBIT建立了双层源项模型;最后将模型网格导入Fluent软件,建立离散粒子模型,计算得出了1、10、50、100 m粒径的粒子运动轨迹,系统分析了烟云在上升过程中各粒径粒子的分布和运动趋势,给出了不同高度的粒子浓度,为源项分析提供了理论基础。     

岩爆岩石断裂的微观结构形貌分析及岩爆机理

利用扫描电镜(SEM)对岩爆岩石断口微观形貌特征进行研究分析,从微观角度探索岩爆产生的机理。通过对平顶山十二矿岩爆现场取样对其断口形貌特征与地应力和岩石成分之间关系进行研究。巷道围岩劈裂岩块断口形貌多呈台阶状,劈裂面与地应力最大主应力方向平行,岩石断口属拉张断裂,劈裂纹的产生主要是脆性断裂;岩爆抛射出的岩块断口形貌非常复杂,裂面与切应力(最大主应力)方向平行或相交,不同平面内的微裂纹通过与岩爆裂纹间的微裂纹或受撕裂作用形成台阶,表面不平整,属于拉张或剪切型断裂。岩石细观成分对岩爆的影响也较大,结晶程度高、结构致密的硬脆岩石更易发生岩爆。     

Numerical study of particulate suspension flow through wavy‐walled channels

The particulate suspension flow in a channel whose walls describe a travelling wave motion is examined numerically. A perturbation method is employed and the primitive variables are expanded in a series with the wall amplitude as the perturbation parameter. The boundary conditions are applied at the mean surface of the channel and the first‐order perturbation quantities are numerically determined by solving the governing system of ordinary differential equations by shooting technique. The present approach does not impose any restriction on the Reynolds number of the flow and the wave number and frequency of the wavy‐walled channel, although it is limited by the linear analysis. The wall shear stress and the positions of flow separation and reattachment points are computed and the influence of the volume fraction density of the particles is examined. The variations of velocity and pressure of the particulate suspension flow with frequency of excitation are also presented. Copyright © 2005 John Wiley & Sons, Ltd.